Michigan's ban on gay marriage is unconstitutional, a federal judge said on Friday as he struck down a law that was widely embraced by voters a decade ago – the latest in a recent series of decisions overturning similar laws across the country.

US district Judge Bernard Friedman announced his ruling after a rare two-week trial that mostly focused on the impact of same-sex parenting on children.

There was no indication that the judge was suspending his decision while the state can appeal. The decision was released shortly after 5pm, when most county clerk offices in Michigan were closed. It was not clear if gay marriages could begin immediately.

Seventeen states and the District of Columbia issue licenses for same-sex marriage. Since December, bans on gay marriage have been overturned in Texas, Utah, Oklahoma and Virginia, but appeals have put those cases on hold.

Two Detroit-area nurses, Jayne Rowse and April DeBoer, want to get married, but the original purpose of their 2012 lawsuit was to overturn Michigan's ban on joint adoptions by same-sex couples. They are raising three adopted children with special needs at their Hazel Park home. But they cannot jointly adopt each other's children because joint adoption in the state is tied exclusively to marriage.

Rowse, 49, and DeBoer, 42, didn't testify, and the trial had nothing to do with their relationship. In fact, attorneys for the state told the judge that they are great parents.

Instead, the state urged the judge to respect the results of a 2004 election in which 59% of voters said marriage in Michigan can only be between a man and a woman. Conservative scholars also questioned the impact of same-sex parenting on children.

But experts testifying for Rowse and DeBoer said there were no differences between the children of same-sex couples and the children raised by a man and woman. And the University of Texas took the extraordinary step of disavowing the testimony of sociology professor Mark Regnerus, who was a witness for Michigan.